1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an array substrate for an in-plane switching liquid crystal display (IPS-LCD) device and a method of fabricating the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The liquid crystal molecules have long, thin, shapes, and have an initial alignment direction including initial pretilt angles. The alignment direction can be controlled by applying an electric field to influence the alignment of the liquid crystal molecules. Due to an optical anisotropy property of liquid crystal, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image having a desired brightness can be produced.
Among the known types of liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images.
Liquid crystal display (LCD) devices include two substrates spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. In one type of LCD device, each of the substrates includes an electrode with the electrodes of each substrate facing each other. A voltage is applied to each electrode inducing an electric field between the electrodes. The arrangement of the liquid crystal molecules is changed by varying the intensity of the electric field.
Because the electrodes are positioned respectively on each of the two opposing substrates, the electric field induced between the electrodes is perpendicular to the two substrates. Accordingly, LCD devices of this type have a narrow viewing angle because of the vertical electric field.
In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. An IPS-LCD device includes a pixel electrode and a common electrode on the same substrate.
FIG. 1 is a cross-sectional view illustrating an IPS-LCD device according to the related art. The IPS-LCD device includes an array substrate and a color filter substrate with a liquid crystal layer interposed therebetween.
More particularly, as shown in FIG. 1, a pixel region “P” is defined on a first substrate 50. A thin film transistor “T” is formed in the pixel region “P” on the first substrate 50 for use as a switching element. Common electrodes 58 and pixel electrodes 72 are also formed in the pixel region “P.” The thin film transistor “T” includes a gate electrode 52, a semiconductor layer 62, a source electrode 64, and a drain electrode 66. The common electrodes 58 alternate with and are parallel to the pixel electrodes 72 on the first substrate 50. The common electrode 58 is formed of the same material and on the same layer as the source and drain electrodes 64 and 66. The pixel electrode 72 is formed of a transparent conductive material. In addition, a common line connected to the common electrodes 58 is formed on the first substrate 50.
A second substrate 30 is spaced apart from the first substrate 50. A black matrix 32 is formed on an inner surface of the second substrate 30 facing the first substrate 50. The black matrix 32 on the second substrate 30 corresponds to the thin film transistor “T,” the gate line and the data line on the first substrate 50. A color filter layer 34 including three color filters of red 34a, green 34b, and blue (not shown) is formed on the black matrix 32. The color filter layer 34 corresponds to the pixel region “P” on the first substrate 50.
A liquid crystal layer “LC” is interposed between the first substrate 50 and the second substrate 30. The alignment of the liquid crystal layer “LC” is controlled by a horizontal electric field 95 induced between the common electrode 58 and the pixel electrode 72.
FIG. 2 is a plan view illustrating an array substrate for an in-plane switching liquid crystal display (IPS-LCD) device according to the related art. The array substrate of FIG. 2 includes a common electrode and a pixel electrode formed of a transparent conductive material.
In FIG. 2, a gate line 54 and a data line 68 are formed on a substrate 50. The gate line 54 and the data line 68 cross each other to define a pixel region “P.” A common line 56 is spaced apart from and parallel to the gate line 54. A thin film transistor “T” is formed at a crossing portion of the gate line 54 and the data line 68. The thin film transistor “T” includes a gate electrode 52, a semiconductor layer 62 on the gate electrode 52, a source electrode 64 and a drain electrode 66. The gate electrode 52 is connected to the gate line 54 and the source electrode 64 is connected to the data line 68.
Common electrodes 90 and pixel electrodes 92, which are parallel to and spaced apart from each other, are formed in the pixel region “P.” The common electrodes 90 contact the common line 56 and extend into the pixel region “P.” The pixel electrodes 92 contact the drain electrode 66 and extend into the pixel region “P.”
When one of the common electrodes 90 and the pixel electrodes 92 is formed of a transparent conductive material, the IPS-LCD device may exhibit image deterioration effects such as stains.
FIGS. 3A and 3B are cross-sectional views, which are taken along a line “III-III” of FIG. 2, showing an exposure step for forming common electrodes and pixel electrodes of an array substrate for an in-plane switching liquid crystal display (IPS-LCD) device according to the related art. As shown in FIG. 3A, a gate insulating layer “GL” is formed on a substrate 50 and a data line 68 is formed on the gate insulating layer “GL.” In addition, a passivation layer “PL” is formed on the data line 68 and a transparent conductive material layer 96 is formed on an entire surface of the passivation layer “PL.” A photoresist (PR) layer 98 is formed on the transparent conductive material layer 96. A mask “M” having a transmissive portion “F1” and a blocking portion “F2” is disposed over the PR layer 98. The mask “M” is aligned with the substrate 50 such that the blocking portion “F2” corresponds to common electrodes and pixel electrodes.
The substrate 50 is disposed on a chuck “CK” of an exposure apparatus and light “L” is irradiated onto the PR layer 98 through the mask “M.” Since the chuck “CK” is formed of a metallic material, a part of the light “L” passing through the mask “M,” the PR layer 98, the transparent conductive material layer 96, the passivation layer “PL” and the gate insulating layer “GL” is reflected by the chuck “CK.” The part of the light “L” reflecting from the chuck “CK” is irradiated onto the PR layer 98 corresponding to the blocking portion “F2” of the mask “M.” The undesired irradiation of the PR layer 98 due to reflection from the chuck “CK” creates non-uniformities in PR patterns.
As shown in FIG. 3B, after the light “L” (of FIG. 3A) is irradiated, the PR layer 98 (of FIG. 3A) is developed to form a PR pattern (not shown). The transparent conductive material layer 96 (of FIG. 3A) is etched using the PR pattern as an etch mask to form common electrodes 90 and pixel electrodes 92. Since the intensity of light irradiated onto the PR layer 98 (of FIG. 3A) corresponding to a portion “K” over the chuck “CK” (of FIG. 3A) is higher than the intensity of light irradiated onto other portions of the PR layer 98 (of FIG. 3A), a first width “d1” of the common electrodes 90 and the pixel electrodes 92 corresponding to the portion “K” is smaller than a second width “d2” of the common electrodes 90 and the pixel electrodes 98 corresponding to other portions. The difference in widths “d1” and “d2” is a critical dimension (CD) deviation of the PR pattern. The CD deviations result in brightness differences in the LCD device that are recognized as stains by LCD device users. Stains caused by chuck reflection may be referred to as chuck stains.
FIG. 4 is a cross-sectional view showing common electrodes and pixel electrodes of an array substrate for another related art in-plane switching liquid crystal display (IPS-LCD) device. In FIG. 4, common electrodes 90 are formed of an opaque material and pixel electrodes 92 are formed of a transparent material. Even though no CD deviation is created corresponding to the opaque common electrodes 90, the transparent pixel electrodes 92 have a CD deviation such that a first width “d1” of the pixel electrodes 92 corresponding to a portion “K” over a chuck is smaller than a second width “d2” of the pixel electrodes 98 corresponding to the other portion.
The common electrodes 90 are simultaneously formed with a gate line (not shown) and a gate electrode. The gate line is formed of a material having a relatively great thickness providing a corresponding relatively low resistivity to prevent a signal delay. Because the common electrodes 90 are created simultaneously with the gate line, the common electrodes 90 also have a relatively great thickness causing a height or step difference in the upper layers formed over the common electrodes 90. When the common electrodes 90 and the pixel electrodes 92 are formed of a transparent material, the step difference causes an observable image imperfection or step stain in the LCD device.
Furthermore, the transparent conductive material has a higher resistivity than the opaque metallic material. Accordingly, in a large size LCD device, the resistivity of a transparent conductive material may cause problems such as a signal delay, and may reduce design freedom when creating large sized LCD devices having pixel electrodes and common electrodes of a transparent conductive material.